1. Field of the Invention
The present invention relates to a semiconductor device used as a high-frequency device and a manufacturing method of the same.
2. Description of the Related Art
An output transistor for use at high frequency, which is intended for a portable cellular phone, a base station of a wireless LAN and the like has been developed conventionally. For example, GaAsFETs are frequently used as a high-frequency output transistor of a microwave band from 500 MHz to 5 GHz, but with the recent advancement of the silicon LSI technique, they are being replaced with so-called silicon LDMOS (Laterally Diffused MOS) transistors which are at lower cost with higher quality than the GaAsFETs.
A schematic constitution of a conventional LDMOS transistor is shown in FIG. 6.
As shown in FIG. 6, electrode-shaped polysilicon films 111 and W silicide films 112 on the polysilicon films 111 are formed by patterning above a p−/p+/p− type silicon semiconductor substrate 101 via a gate insulating film 102, and an interlayer insulating film 103 is formed to cover them. Source diffusion layers 104 and an n+ drain contact layer 105 formed by introducing an n-type impurity are formed in a surface layer of the semiconductor substrate 101, and n− drift layers 106 connected to the n+ drain contact layer 105 to secure high frequency tolerance are formed between the n+ source diffusion layers 104 and the n+ drain contact layer 105. Further, p− channel diffusion layers 107 and substrate contact diffusion layers 108 connected to the p− channel diffusion layers 107 are further formed in this surface layer to cover the n+ source diffusion layers 104.
Openings 109 and 110 are formed in the interlayer insulating film 103. The opening 109 is formed so that a part of a surface of the n+ drain contact layer 105 is exposed, and the opening 110 is formed so that a part of a surface of the source diffusion layer 104 and a part of a surface of a substrate contact diffusion layer 108 are exposed. A drain electrode 121 formed by electrically connecting the opening 109 to the embedded n+ drain contact layer 105 via an underlying film 113 on the interlayer insulating film 103, source electrodes 122 each formed by electrically connecting the opening 110 to the embedded source diffusion layer 104 via the underlying film 113 on the interlayer insulating film 103, and upper electrodes 123 each formed by being electrically connected to the W silicide film 112 and the polysilicon film 111 via the underlying film 113 on the interlayer insulating film 103 are provided, and thereby the LDMOS transistor is constituted. Here, aluminum or an aluminum alloy is used as a material of the drain electrode 121, the source electrodes 122 and the upper electrodes 123, and gate electrodes 124 are each constituted of the polysilicon film 111, the W silicide film 112 and the upper electrode 123.
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-94054
[Non Patent Document 1] Hiroshi Horie, Masahiko, Imai, Akio Ito, and Yoshihiro Arimoto: “Novel High Aspect Ratio Aluminum Plug for Logic/DRAm LSIs Using Polysilicon-Aluminum Substitute (PAS)”, IEDM96, p. 946, (1996)
[Non Patent Document 2] Hiroshi Horie, Masahiko Imai, Akio Ito, and Yoshihiro Arimoto: “Microscopic Wiring Technique by Polysilicon-Aluminum Substitute”, The Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE, SDM96-208 (1997)
[Non Patent Document 3] “Priority Task on Power Device for 21th Century”, Technical Report of the Institute of Electrical Engineers of Japan No. 666, p.36 (1998)
[Non Patent Document 4] M. Shindo, M. Morikawa, T. Fujioka, K. Nagura, K. Kurotani, K. Odaira, T. uchiyama, and I. Yoshida: “High Power LDMOS for Cellular Base Station Applications”, ISPSD 2001, p. 107 (2001)
The performance indicators of the output transistor used for high frequency are maximum transmission frequency fmax predicting the high-frequency operation limit and power gain. In order to increase the fmax, it is necessary to reduce gate parasite resistance, and in order to increase the power gain, it is necessary to reduce parasite capacity Cgd between the gate electrode and the drain electrode.
In the high-frequency power amplifier, it is necessary to increase power of radio waves to be outputted with respect to power applied to the amplifier in order to generate more effective radio waves. This is an extremely important task especially for portable cellular phones using batteries as their power sources. In order to enable a high-frequency operation with high efficiency, it is necessary to completely reduce the parasite resistance and the parasite capacity. A cutoff frequency fT being a performance indicator strongly depends on the parasite capacity as shown by equation (1). The maximum transmission frequency fmax depends on the gate parasite resistance as shown by equation (2).fT=gm/{2π(Cgs+Cds)}  (1)(gm: mutual conductance, Cgs: gate-source parasite capacity, Cds: drain-source parasite capacity)fmax=fT/{2(Rg·Gd)½}  (2)(Rg: gate parasite resistance, Gd: drain conductance)
The parasite capacities Cgs and Cds inhibit the cutoff frequency fT, and the gate parasite resistance Rg inhibits the maximum transmission frequency fmax.
Since the gate structure in which the polysilicon film 111 and the W silicide film 112 are laid on each other is adopted in the prior art, as shown in FIG. 6, the gate parasite resistance Rg is about 10 Ω/square, and reduction in this gate parasite resistance Rg is limited. The gate parasite resistance Rg becomes more noticeable as the gate length is made shorter, and reduction of this is the task for a high-frequency operation. Further, the fact that a depletion layer extends to the polysilicon side of the gate electrode and the mutual conductance gm is reduced also inhibits the high-frequency operation.
The output transistor for use at high frequency is demanded of reduction in the gate parasite resistance to its extreme limit for the purpose of achieving high performance, but even the LDMOS transistor developed to realize this has the problem as described above.